[m-rev.] for review: improve dir./

[Zoltan Somogyi]

This diff moves closer to the scheme I outlined a few days ago.

For review and trying out by Julien on Windows, though I would
also like Peter's opinion on the questions below.

Is the new predicate that this diff exports from dir.m generally useful?
If yes, I will add an entry to NEWS; otherwise, I will move its declaration
to the implementation section.

The comment now in the body of that predicate is a copy of the comments
on the two pieces of code that the new predicate factors out. Can someone
propose a better, more unified wording for it?

Can the documentation of that predicate and the other new internal
preds be improved, and if so, how?

Zoltan.
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